1. Field of the Invention
This invention relates to magnetic-based mechanical translators, and more particularly to mechanical translators with ferrofluid bearings.
2. Description of the Related Art
Ferrofluids are dispersions of finely divided magnetic or magnetizable particles, generally ranging between about 30 and 150 Angstroms in size, dispersed in a liquid carrier. The magnetic particles are typically covered with surfactants or a dispersing agent. The surfactants assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal forces and magnetic interaction, and also provide a chemical composition on the outer layer of the covered particles which is compatible with the liquid carrier and the chemicals in the surrounding environment. Ferrites and ferric oxides employed as magnet particles offer a number of physical and chemical properties to the ferrofluid, including saturation magnetization, viscosity, magnetic stability and chemical stability. Several types of ferrofluids are provided by Ferrotec (USA) Corporation of Nashua, N.H. A summary of patents related to the preparation of ferrofluids is provided in U.S. Pat. No. 6,056,889, while the use of ferrofluid bearings in a moving magnet electrical generator is discussed in copending patent application Ser. No. 10/078,724, entitled xe2x80x9cElectrical Generator With Ferrofluid Bearingsxe2x80x9d, filed on the same day as the present invention by Jeffrey T. Cheung and Hao Xin, and also assigned to Innovative Technology Licensing, LLC, the assignee of the present invention. The contents of this copending application are hereby incorporated herein by reference.
A ferrofluid""s frictional coefficient is roughly related to its viscosity (measured in centipoise (cp)), but not directly. For example, a ferrofluid with a viscosity of 300 cp has been found to have a static friction coefficient of about 0.015, the EFH1 ferrofluid from Ferrotec (USA) Corporation has a viscosity on the order of 6 cp and a static friction coefficient of about 0.002, but a water based ferrofluid with a viscosity of 5 cp has been found to have a static friction coefficient of about 0.01. The higher friction coefficient for the somewhat lower viscosity composition has been attributed to surface tension associated with a water based solvent.
Low friction systems can have other problems. For example, depositing a thin film over a substrate by vapor deposition is performed in a vacuum chamber by rotating and translating the substrate inside the chamber to achieve uniform deposition over a large area. The mechanism that controls this motion is cumbersome, consisting of gears, chains, and both rotary and translational motion vacuum feedthroughs. A simpler less expensive motion control for the substrates would be highly desirable.
Traditional lubricants employed to assist movement of a load bearing mechanism on a substrate, while considerably reducing the frictional forces resisting motion, still involve a considerable amount of friction. For example, whereas the static coefficient of friction between unlubricated hard steel surfaces is typically about 0.6, the corresponding coefficient is about 0.08-0.1 for vegetable and animal oil lubricants, 0.14-0.2 for mineral oils, 0.12 for graphitised oils, 0.1 for molybdenum disulfide, 0.08 for oleic acid, 0.4 for alcohol and benzene, and 0.2 for glycerine. Even lower frictional coefficients, making it easier to translate a load, would be desirable.
The present invention provides a mechanical translator system with an ultra low degree of friction, and a novel ferrofluid composition that can be used as a lubricant for the translator. The term xe2x80x9ctranslationxe2x80x9d as used herein includes both changes of position and/or rotation. In a preferred embodiment the translator includes a moveable magnet that is arranged to carry a load and is disposed for movement on a substrate. Its magnetic axis is generally transverse to the substrate, such that its magnetic field has a maximum density adjacent the substrate. A ferrofluid lubricant is preferably provided between the magnet and substrate. A ferrofluid composition that can be used to achieve a particularly low friction has a viscosity substantially less than 5 cp. It consists of a light mineral oil medium mixed with isoparaffinic acid, with the ratio of isoparaffinic acid to light mineral oil preferably in the range of about 2:1 to about 4:1. The resulting coefficient of static friction has been found to be in the approximate range of 0.0008-0.0012.
A plurality of moveable magnets, each with a magnetic axis generally transverse to the substrate, can be used to support a platform upon which a load can be placed, or which itself can comprise a load. The magnets are preferably connected to respective fixed locations on the platform, which can be formed from a magnetic material, or a nonmagnetic material which might include an alloy that shields magnetic fields. A ferrofluid lubricant is provided between the magnets and substrate for ultra low frictional movement. A ferrofluid with a low vapor pressure should be selected for environments in which other ferrofluids tend to dry out.
When applied to vapor deposition apparatus, the moveable magnets are disposed on the floor of a vacuum chamber which includes a vapor source for depositing a thin film on a substrate carried by the platform. A controller formed from a magnetic material outside the vacuum chamber, on the opposite side of the floor from the moveable magnets, controls the movement of the magnets and platform to achieve uniform deposition, eliminating the need for the cumbersome control equipment and its mechanical communication through the vacuum chamber that were previously required.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.